This invention is in general directed to a method for working hypoeutectoid steels within a temperature range to effect a desired reduction in cross-sectional area. The hypoeutectoid steels can be heated to a temperature for a time to obtain optimum hardness and ductility suitable for cold-working. More specifically, the invention is directed to a method for working and heat treating hypoeutectoid steels, for example, steels containing about 0.30 to about 0.80% carbon, wherein said steels are worked to reduce the cross-sectional area by not less than 60% within a temperature range of about the A.sub.1 temperature to about 150.degree. F. below the A.sub.1 temperature. The steels are subsequently heated to about the A.sub.1 temperature to obtain a hardness and ductility suitable for cold-working, such as cold-heading and the like.
Steels used for cold working into various products, such as bolts, screws and the like, generally are of the hypoeutectoid type containing up to about 0.80% carbon.
Prior art practice in manufacturing cold-workable hypoeutectoid steels is to refine the steel in a metallurgical furnace, such as an electric furnace and the like, tap and teem the steels into ingot molds to form ingots. The ingots are hot-worked at an austenitizing temperature into the end product, such as bars, billets, rods, wire and the like. The end product is slow cooled to ambient temperature. The steels are spheroidize annealed to obtain a uniform structure which is substantially completely spheroidized; is substantially completely free of a carbide network and pearlite; and is relatively soft and ductile. Because of the duplex structure and high hardness of the steels after slow-cooling from the hot-working temperatures, the spheroidize anneal cycles which are used are lengthy. The steels must be soaked at the proper temperature for from about 15 hours to days to completely and effectively produce the desired microstructure and hardness and ductility required for good cold formability. In order to decrease the time of annealing, the steels are alternately heated and cooled to a few degrees of temperature above and below the A.sub.1 temperature several times. Although the microstructure and hardness produced by the practices are acceptable by present day standards, the cyclic heating and cooling practice does not effectively reduce the length of the annealing cycle. Then, too, the microstructure of hypoeutectoid steels treated by the cyclic heating and cooling method can contain evidence of lamellar carbides. The hardness of the steels is reduced to a hardness suitable for cold-working after lengthy time at the spheroidize annealing temperature but maximum reduction in hardness may not be achieved.
Recently several improvements in the manufacture of cold-workable steels have been suggested. One such improvement is U.S. Pat. No. 3,285,789 issued Nov. 15, 1966 to Raymond A. Grange et al. titled "Method of Softening Steel." The improvement is directed to heating hypoeutectoid steels to temperatures wherein the steels are completely austenitic, working the steels while at these temperatures, cooling the steels to ambient temperature and spheroidize annealing the worked steels within a temperature range between the A.sub.1 temperature to 50.degree. F. below the A.sub.1 temperature. The structure obtained after spheroidize annealing is satisfactory by present standards. Complete spheroidization and the elimination of lamellar carbides is not achieved in the as-worked condition.
The as-worked steels contain ferrite and pearlite, therefore the steels must be spheroidize annealed to produce a spheroidized structure.
Another improved method is directed to hypereutectoid steels and highly alloyed steels as described in U.S. Pat. No. 3,459,599 issued Aug. 5, 1969 to Raymond E. Grange titled "Method of Thermomechanically Annealing Steel." Hypereutectoid steels are heated to and drastically worked at a temperature not more than 150.degree. F. above the A.sub.1 temperature and .[.ore.]. .Iadd.are .Iaddend.finished below the A.sub.1 temperature but not more than 50.degree. F. below the A.sub.1 temperature. The method, while applicable to hypereutectoid steels, is not applicable to hypoeutectoid steels. The problems of complete spheroidization with elimination of lamellar carbides connected with hypoeutectoid steels is not solved.
Although prior art practices have been developed to roll steels ferritically as disclosed in U.S. Pat. No. 3,076,361 issued Feb. 5, 1963 to S. Epstein et al. titled "Rolling Steel in Ferritic State," the high alloy and tool steels to which the process is directed, must be heat treated, for example, spheroidize annealed, prior to heating and working. The special heat treatment prior to heating for working and controlled heating for working increase the cost of the production of the steel.